1. Field of the Invention
The present invention relates to a lighting device for a non-self-emitting display such as a small-size liquid crystal display used in a cellular phone, a PDA and so on.
2. Description of the Related Arts
In recent years, such a lighting device has employed a light emitting diode (hereinafter abbreviated as “LED”) for a light source thereof.
FIG. 6a and FIG. 6b show a typical type of the lighting device with LEDs. The lighting device comprises a plurality of LED units 64-1, 64-2, 64-3 and 64-4, all of which are placed adjacent to a side edge surface 63 of a rectangular light guide plate so that light exiting from the LED unit 64-1, 64-2, 64-3 and 64-4 enters the light guide plate 10 through the side edge surface 63. The light guide plate 10 includes reflective prisms 62 formed on a bottom surface thereof which are successively arranged from the side edge surface 63 to the opposite side edge surface 65.
As shown in FIG. 6b, there are provided a reflecting sheet 16 below the light guide plate 10 and a diffusion sheet 70 and prism sheets 68 and 66 above the same and all of those elements are accommodated in a housing 60. A non-self-emitting display such as a liquid crystal display is placed on top of the housing.
FIG. 7a illustrates a behavior of the light for illumination. A light 15 from the LED unit 64 that has entered the light guide plate 10 advances in the light guide plate 10 while repeatedly bouncing between reflective prisms 62 arranged in the lower surface of the light guide plate and an upper surface of the light guide plate. During the repeated bouncing, an angle of incidence of the light impinging on the upper surface of the light guide plate is gradually made small and finally become smaller than a critical angle, thereby allowing the light to exit upward through the upper surface of the light guide plate. In addition, the light that may exit from the lower surface of the light guide plate is to be reflected by the reflecting sheet 16 and returned to the light guide plate 10. The light exited from the light guide plate 10 may be diffused by the diffusion sheet 70 and then directed toward a non-self-emitting display 74 by the prism sheets 68 and 66 so as to illuminate the display.
It is to be noted that reference numeral 20 designates a flexible printed circuit board serving for establishing an electric connection between the LED unit and an external device.
Referring now to FIGS. 8a and 8b, coordinate axes used in the present specification are illustrated. Z-axis extends from an LED unit 64, which comprises an emission section 76 and a substrate section 12, toward the light guide plate 10 and perpendicularly to the side edge surface 63 of the light guide plate 10, X-axis is orthogonal to the Z-axis and extending in parallel to the side edge surface 63 of the light guide plate 10, and Y-axis is orthogonal to the Z-X plane.
Further, referring to FIG. 8b, with regard to the light emitted from the LED unit 64, an angle between a component X′ of the light projected onto the X-Z plane and the Z-axis and an angle between a component Y′ of the light projected onto the Y-Z plane and the Z-axis are both denoted by θ which will be referred to as “light emission angle” hereinunder.
FIG. 7b is a diagram representing the directivity of the light emitted from the LED unit as shown in FIG. 7a. In the diagram, the lateral axis represents the light emission angle θ, and the vertical axis represents ratio of the light intensity of the components of the light projected on the X-Z plane and the Y-Z plane wherein the ratio of the component of the light emission angle θ equal to zero is defined as 1. As can be seen from the diagram, the profiles of the ratios of the light intensity of the components are substantially the same and therefore represented by a single curve.
FIG. 9a is a perspective view of the LED unit 64, FIG. 9b shows the LED unit 64 viewed from the right side along the Z-axis, FIG. 9c is a sectional view of the LED unit 64 in an approximately central region taken along a plane (horizontal plane) parallel to the X-Z plane, and FIG. 9d shows the LED unit 64 viewed from the left side along the Z-axis.
The substrate section 12 comprises a substrate 26 having internal terminals 32 and external terminals 34 which are electrically connected by vias 36 formed in the substrate 26. The emission section 76 comprises a light shielding wall 78 in the shape of a rectangular cylinder and a resin block 80 filling up the interior space in the light shielding wall 78. Within the resin block 80 are embedded a LED chip 28 mounted on the substrate 26 and gold wires 30 connecting the internal terminals 32 with the LED chip 28. The resin block 80 has a front surface 77, which faces to the light entering surface of the light guide plate, and thus, the front surface 77 serves as an emission surface for the light emitted from the LED chip 28 toward the light guide plate.
With reference to FIG. 10a, a diffused light emitted from the LED unit 64 is as indicated by a shaded triangle 22 enters the light guide plate 10 and contributes as the light for illumination. However, other light diffused outside the shaded triangle 22 will not enter the light guide plate 10, thereby not being used for illumination. FIG. 10b shows such a matter by using the same diagram as that of FIG. 7b, i.e., the light indicated by shaded portions 78, 80 is not be used for illumination.
The quantity of the light that cannot be used for illumination may increase or decrease in dependence on a distance L1, between the LED unit 64 and the light guide plate 10 and a thickness T, of the light guide plate as shown in FIG. 10a. 
If the distance L1 is made zero, all of the light could be taken into the light guide plate. However, in actuality, it is impossible to make the distance L1 zero in the lighting device.
In addition, if the thickness T of the light guide plate is made thicker, more light could be taken into the light guide plate to be used for illumination. However, the thickness T is not allowed to be increased for the recent trend of a lower profile of the lighting device, which has been desired in association with the demand for a low-profile, lighter portable device. Actually, it is said that in order to reduce the thickness of the lighting device by 20% to 30%, the thickness of the light guide plate is needed to be reduced by around half.
Unfortunately, as described with reference to FIG. 10, if the thickness of the light guide plate is decreased, less light is taken into the light guide plate.
Accordingly, although there is the trend to make the lighting device thinner, it will deteriorate the emission efficiency of the light source, and, therefore, it is impossible to establish the coexistence of “making thinner” and “making brighter” in the lighting device.
In connection with the lighting device with LEDs, there has been a proposal for solving a problem inherent to the lighting device that there appear dark regions or low brightness regions on the light exiting surface of the light guide plate at locations between the LEDs and adjacent to the side edge surface of the light guide plate along which the LED units are arranged. According to the proposal, the upper and lower surfaces of the emission section of the LED unit are mirror-finished or provided with light shielding layers made from material having a high reflectance without subjecting the left and right side surfaces to such treatment (see, for example, Japanese Patent Laid-open Publication No. 2004-127604). The treatments as stated above are not easy but expensive.